Channel die tests on Al and Cu polycrystals: Study of the prestrain history effects on further large strain texture

Al and Cu polycrystals have been prestained in a channel die to a compressive strain of 0.04-0.08 (small prestrain) and 0.40-0.55 (large prestrain) and then secondarily strained in different orientations—still in the channel die—up to a second strain of 0.75 in compression. The polycrystal hardening anisotropy is measured at small and large strain and reported under a form similar to the latent hardening ratio curves for single crystals, say here the ratio of the initial (back extrapolated) yield stress for the secondary straining to the maximum stress at the end of the prestrain. Texture measurements have been performed at the end of the secondary straining for the copper samples and are reported under the form of {111}, {200} and {110} pole figures. The observed differences on the measured textures are discussed with regard to the prestrain orientation and amplitude with special attention to the associated polycrystal hardening anisotropy and its more probable causes, geometrical or microstructural. The main observations are: (1) for copper, there is a clear hardening anisotropy after small strains, which does not seem due to texture because of too mild associated rotations, but more reasonably to an intracrystalline “latent hardening” effect in polycrystals. This effect appears greater in Cu than in Al in agreement with theoretical expectations (from the stacking fault energy of the two materials) and experiments on single crystals confirming the above interpretation. This hardening anisotropy depends on the way the load path is changed and differs for the various secondary orientations; (2) a weak hardening anisotropy occurs after large strains where one can expect more effect of the associated texture, which here appears to act in the opposite direction of that due to the latent hardening; (3) the correlation between changes on deformation textures at large strain and path changes during the sample loading appears as follows: while a path change after a large prestrain has a strong effect on the texture pattern after a secondary strain of 0.75 both on intensity and location of the maxima, it has only a mild effect after a weak prestrain but still showing noticeable differences for some orientations on the intensity maxima, which are the orientations associated with the highest measured hardening values at the end of the prestrain.     

Effect of prestrain and baking on the secondary deformation properties of cold-rolled ultra-high-strength steel sheets

Two experimental steels with tensile strength above 980 MPa were prepared to investigate the effect of prestrain and baking on their mechanical and fracture behaviors. The experimental results reveal that,for both experimental steels,with increases in the prestrain level,the bake hardening value increases before reaching a maximum point,and then decreases with further increases in the prestrain level. The results of a "bending-baking-secondary bending"test indicate that the secondary bendability deteriorates at a high level of prestrain. The yield strength of the experimental steels was found to increase and the elongation to decrease after high levels of prestrain and bake hardening. Fracture morphology images indicate that a high prestrain level is associated with shallow dimples and more and larger local cleavage areas.     

Effect of prestrain on the ductile fracture behavior of an interstitial-free steel

The effect of tensile prestrain on the ductile fracture behavior of an interstitial-free (IF) steel has been studied using primarily (1) the analysis of void density by optical microscopy, (2) characterization of the size of dimples by scanning electron microscope (SEM) and image analyzer, and (3) estimation of strain hardening behavior of a series of prestrained tensile specimens, loaded until fracture. The variation of void density with local plastic strain around the necked region of the specimens indicated the existence of two types of void nucleation pertaining to inclusions and precipitate particles. The critical strain for void nucleation (ε{inn}) for the precipitate particles initially increases and then decreases with the increase in percentage prestrain. This phenomenon has been explained using the strain hardening exponent and nature of dislocation-particle interaction. The nature of variation of the average size of the dimples and that of ε{inn} with prestrain are found to be similar. The dimple size thus bears a proportional relationship with the void, nucleation strain ε{inn} and hence the former can be used to predict (ε{inn}) for IF steel.     

Effect of prestrain on aging and bake hardening of cold-rolled, continuously annealed steel sheets

To understand the bake hardening (BH) behavior in an actual automotive part, 40 tensile specimens were machined from the actual press-formed outer-door panel of a compact car and both bake and work hardenability distribution data were determined. Strain applied by actual press forming was estimated from the work hardening data. Finally, the effects of prestraining mode and amount on ambient aging and BH response were also investigated. The BH widely ranged from 10 to 54 MPa and the work hardenability was between 17 and 82 MPa, depending on panel location. Bake hardening in the outer-door panel decreased as the work hardening increased, indicating that the BH steel must be applied to the shallow drawn parts in order to maximize the BH effect in dent resistance. In order to establish the effects of prestrain and ambient aging time on the age and subsequent BH, the specimens were prestrained and aged at ambient temperature for various time intervals, and then baked at 170 °C for 20 minutes. In the as-temper-rolled and press-formed condition, the steels were extremely resistant to ambient aging. However, it was found that a 0.3 pct tensile prestrain was sufficient to initiate ambient aging within 1 day, and the effect was accelerated with greater prestrain. With 8 days of ambient aging, all prestrained steels exhibited 20 to 25 MPa of age hardening. Irrespective of prestrain amount in the range of 0.5 to 5.0 pct, the BH decreased as the aging time increased.     

Influence of elastic inclusion morphology and matrix hardening behavior on bauschinger effect in metal matrix composites

Finite element modeling based on axisymmetrical cells was performed for relating the Bauschinger effect (BE) in metal matrix composites (MMCs) to the reinforcement volume fraction and shape, matrix hardening behavior, and plastic prestrain levels. It was found that elastic inclusions in MMCs introduce a significant BE, which is ascribed to the residual phase stresses in the component phases. The BE introduced by cylindrical inclusions is more significant than that of spherical ones and increases rapidly with increasing the aspect ratio of the cylindrical inclusion. The volume fraction of the elastic inclusion has a strong effect on BE. The hardening behavior of the matrix has a weak effect on BE. The BE increases with increasing plastic prestrain level.     

Influence of PreTransformed Martensite on WorkHardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

 The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work hardening behavior under the uniaxial tensile condition. The X ray diffractometer (XRD) was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work hardening curve of prestrained specimens observably changes with true strain. The work hardening exponent n of stainless steel decreases with the increase of pre transformed martensite. The achievement is a significant contribution to the process design during pressing.     

Influences of cell walls and grain boundaries on transient responses of an if steel to changes in strain path

The effects of changes in strain path on plastic behaviour in sheets of an interstitial-free steel with two widely different grain sizes were investigated. The sheets were prestrained in rolling and, apart from supplementary tests, they were tested in uniaxial tension at 90° to the rolling direction. The results support the following conclusions. The magnitude of the increase in reloading yield stress and amplitude of the subsequent reduction in work hardening depend on the strength of dislocation walls generated in the prestrain rather than the grain size. The walls are more effective barriers to dislocation glide in freshly activated slip systems than to glide in the original slip systems operating in the prestrain. The primary cause of the subsequent reduction in hardening rate is disruption and partial dissolution of the original dislocation substructure. The final recovery in hardening rate is caused by generation of a new substructure compatible with the new deformation mode.     

Plastic behavior of aluminum-killed steel following plane-strain deformation

Aluminum-killed steel sheets have been subjected to plane-strain prestrain in three ways: two-pass rolling, multi-pass rolling, and inplane, plane-strain tension. Subsequent uniaxial tensile tests were performed to evaluate the residual work-hardening behavior. The subsequent hardening curves depended primarily on the relative direction between major strain axes in the two deformation stages and very little on the specific prestrain procedure. These curves showed high initial yield stresses followed by a region of low (or negative) work hardening rate. This behavior contrasted with earlier results for 70/30 brass sheet, and a model of subsequent tensile behavior based on a strain-induced stress transient emerged. Formerly Staff Research Scientist, General Motors Research Laboratories     

Examination of applied stress and prestrain effects on recovery of work hardened Cu-SiO2

Effects on external stress and prestrain on recovery of work hardening have been studied in Cu-SiO₂ crystals. When the external stress is below a critical value, approximately equal to the yield stress, the softening on annealing is independent of the external stress. The softening decreases when the external stress exceeds the critical value. The temperature dependence of the softening rate is insensitive to the amount of the prestrain which has caused work hardening. It is concluded that the interaction between Orowan loops surrounding a SiO₂ particle is not so strong as to affect the climb rate of individual Orowan loops considerably.     

Influence of prestrain history on fracture toughness properties of steels

The effects of prestrain history on fracture toughness properties (J _Ic values andJ _R curves) of 4340 steel and 316 stainless steel were investigated. It was observed that monotonic prestrain decreased fracture toughness of both steels regardless of prestrain level. Although cyclic prestrain elevated fracture toughness of 4340 steel, it degraded that of 316 stainless steel. The effects of cyclic prestrain on fracture behavior of 4340 steel and 316 stainless steel were found to be related to cyclic softening and cyclic hardening characteristics, respectively. Moreover, material strengths rationalized the influence of prestrain history on fracture toughness properties of these two steels. Formerly with the Westinghouse Electric Corporation     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

Latent hardening behavior of monocrystalline Al-Mg solid solution

The latent hardening behavior of dilute Al-Mg single crystal was investigated in this study. We performed the latent hardening tests on five systems, one in each of the five system groups. The latent hardening ratios (LHR) and the hardening rates were calculated. The LHR of systems that form attractive junctions is highest in this investigation. The LHRs of systems that form Lomer-Cottrell sessile locks, Hirth locks, or cross-slip systems are in the middle range. The coplanar system has the lowest LHR, which is in agreement with the theoretical prediction. An equation was developed that correlates the LHR with the dislocation densities at various prestrain values. The secondary deformation curve is predicted qualitatively in accordance with the interaction strength of the latent system with the primary system. Based on such a model, a prediction of the shapes of the secondary deformation curves in the strongest and weakest latent systems can be made.     

The influence of biaxial prestrain on the tensile properties of three aluminum alloys

We have investigated the influence of planar biaxial prestrain on the subsequent tensile behavior of three aluminum alloys. The flow stability encountered in the uniaxial stage is explained in terms of the reduced tensile hardening following biaxial prestraining. A critical stress value approximately equal to the nonprestrained ultimate tensile strength is shown to be the controlling factor for the onset of failure.     

Fatigue of a dual- phase steel

A study has been made of the fatigue of a V containing dual-phase steel, whose tensile strength is equivalent to that of SAE 980X high strength, low-alloy (HSLA) steels, as a function of prestrain. It is found that the cyclic stress-strain curve, strain-life response and notch sensitivity are little affected by pre-strains of up to 8 pct: This is in contrast to monotonie flow strength which increases substantially with prestrain. The fatigue performance of the dual-phase steel, while different in detail from that of other HSLA steels, is intermediate between that for SAE 950X and 980X steels. However, the notch fatigue behavior is equivalent to that of 980X steels. The fatigue response of dual-phase steel can be understood in terms of its high rate of work hardening which is a consequence of its ferrite plus martensite microstructure.     

Plastic deformation and the flow stress of aluminium-lithium alloys

Activation analysis has been used to investigate plastic deformation in Al-Li alloys. The flow stress can be resolved into a component produced by work hardening and a friction stress produced by solid-solution or precipitation hardening. Macroscopic work softening is observed at larger strains in compression tests on precipitation hardened specimens and the results of the analysis indicate that softening is caused by a strain-induced reduction in the friction stress. It is believed that work softening results from the shear of ordered precipitates but an argument is presented to show that macroscopic work softening can only occur when Stage III work hardening is established. At higher temperatures, alloys exhibit the Portevin-Le Chatelier (P-LC) effect in both the solution treated and precipitation hardened condition, but, on the basis of the experimental evidence, it is believed that the detailed mechanism responsible is different in precipitation hardened specimens to that which occurs in the classical P-LC effect, and is associated with the shear of ordered precipitates.     

Mechanical Properties and Bake Hardening Behaviour of two Cold Rolled Multiphase Sheet Steels subjected to CGL Heat Treatment Simulation

In recent years, multiphase steels have become a material of choice for use in the car manufacturing industry owing to their excellent mechanical properties. It is anticipated that in the years to come these steels will show the highest increase in usage. A particular aspect of their potential is that multiphase steels often show good bake hardening (BH) properties. The main factors that govern the microstructures and the properties of these steels are the chemical composition and the production process parameters. In this work two commercial cold rolled sheet steels with different carbon content were investigated. In order to produce dual phase (DP) steels with a ferrite‐martensite microstructure, the as‐received material was subjected to heat treatment simulating continuous galvanising line (CGL) cycles with an overageing zone before the zinc pot. After a first CGL cycle predominantly ferritic microstructures with small amounts of martensite, pearlite and retained austenite were obtained, which resulted in deviations from typical DP properties, e.g. in the occurrence of discontinuous yielding. A higher line speed led to improved mechanical properties. BH prestrain was varied between 0 and 10%. While only very little bake hardening was observed without prestrain, with increasing prestrain the amount of BH was evolving quickly towards larger values of more than 60 MPa. Generally, the BH values were somewhat larger for higher carbon content. Finally, an optimised CGL cycle was simulated at laboratory scale with changes in the process parameters. Thus, characteristic DP microstructures resulting in desired mechanical properties were obtained. For these optimised conditions, BH₂ values in excess of 60 MPa were achieved for both steels investigated.     

Microstructure and texture evolution under strain-path changes in low-carbon interstitial-free steel

The transmission electron microscopy (TEM) microstructure of a low-carbon Ti-killed interstitial-free (IF) steel has been examined after simple-shear/simple-shear and uniaxial-tension/simple-shear strain-path changes, in connection with the crystallographic orientation of the grains. The results are discussed in the context of the inter-relation between the microstructure and texture evolutions and their joint influence on the mechanical behavior. A partial disappearance of prestrain microstructures is shown to cause the stagnation of work hardening at earlier stages of reversed loading during Bauschinger simple-shear sequences. Under progressing reversed deformation, a fragmentation of the grains of unstable orientations still slows down the work-hardening rate. A strong localization of plastic flow within microbands following an orthogonal strain-path change is shown to occur within the grains containing well-developed prestrain dislocation boundaries and belonging to certain orientation groups.     

表层弥散预应变硬化对薄钢板变形局部化的影响

试验研究了具有一定形貌及粗糙度的激光毛面薄钢板及同材质普通薄钢板的失稳行为。结果表明激光毛面薄钢板变形局部化过程的明显滞后于普通板，其表层弥散分布的预应变硬优点在稳定薄板变形，延缓变形局部化过程中起到重要作用。     

Isotropic and kinematic hardening in a dispersion-strengthened aluminum alloy

The room temperature mechanical behavior of a dispersion strengthened aluminum alloy was examined in tension, compression, and in fully reversed loading. The alloy, 8009, is characterized by a high volume fraction of 50–100 nm dispersoid (25%) and 0.5 mm grain size. In tension, 8009 exhibits low strain to UTS and large post uniform elongation; in compression, near steady state deformation is observed after 2–3% strain. The Bauschinger effect was quantified as a function of prestrain in the forward direction. The experimental reverse loading curves were compared to those expected for ideal isotropic hardening and ideal K1 type kinematic hardening. The alloy exhibits nearly pure kinematic hardening of the K1 type. Based on the microstructure and the fully reversed loading behavior, the monotonic deformation behavior is explained.     

Bauschinger Effect in Microalloyed Steels: Part II. Influence of Work Softening on Strength Development During UOE Line-Pipe Forming

The Bauschinger effect (a reduced yield stress at the start of reverse deformation following forward prestrain) is an important factor of strength development for cold metal forming technology. In steels, the magnitude of the Bauschinger effect depends on composition, through the presence of microalloy precipitates, and prior processing, through the size and distribution of the microalloy precipitates and the presence of retained work hardening. In this article, the parameters of the Bauschinger effect and work hardening (coefficient and exponent) in forward and reverse deformations were quantitatively related to the particle number density and dislocation density for two high-strength low-alloy (HSLA) steels. An example of the application of the obtained dependences is discussed with respect to the strength development during UOE forming of large diameter line pipes.